How big of a canvas for an alternative guest book?

We're doing something very similar to that, with the Philadelphia skyline and our names/date personalized on it from an Etsy shop. We're having about 130-150 people and bought the 19" x 27" size. Here is the stock photo for reference. We won't mind if people sign above the skyline too.

We had a 20 x 30 canvas picture of ourselves that people signed. We had 350 guests (including children), and of course not everyone signed it but it was pretty full. I loved doing a canvas instead of a traditional guest book because now I see it everytime I'm in my room.

I did a 24x36 for up to 300 guests

Thanks everyone! I got this great idea I don't know if I'll be able to pull off and it'll definitely require knowing the size too. My fiancé collects movie posters (like rare ones and foreign ones and limited run ones) so I was like why don't I make a one of a kind 1 of 1 limited custom POSTER for people sign as our guest book?!

I have no graphic design skills.

Here we go!

What pen or markers to do you on canvas for signing??